1. Field of the Invention
This invention relates generally to the field of radio turners and more specifically o the field of subsidiary communications authorization radio turners that provides high quality output of sub-carrier signals as used in the commercial FM band. The invention may also be applied wherever sub-carrier signals are used, such as with secondary-audio-product (SAP) audio from television broadcasters, or in main carrier signal detection, where especially high quality is needed.
2. Background Art
Commercial FM radio broadcasters each has just one main channel of programming available at its designated frequency. To allow more than one program to be simultaneously sent by the same radio station, the FCC has authorized sub-carriers to the same designated frequency. This allows secondary programming on the same signal. This programming is restricted in the United States such that normal commercial AM/FM radios can not decode this signal. The sub-carrier programs are typically targeted to ethnic, language, or other special interest type groups. The sub-carrier signal is generated by having the SCA audio programming frequency-modulate the sub-carrier, which is typically at 67 KHz or 92 kHz. The modulated signal is band-with limited to approximately 8-14 kHz. This signal is then attenuated approximately 20 dB and super-imposed to the main audio, which in turn frequency modulates the main carrier in the 88-108 MHz range. Part of the intent is that the SCA signal will not interfere with the main signal. More commonly, it is the main signal that degrades the sub-carrier. The sub-carrier is much smaller, more limited in band-with, imbedded inside a strong signal; and it goes through two modulation phases before transmission. This whole double modulation process makes the SCA signal much more challenging for the tuners in the decoding end.
Relevant technology is found in the use of commercial AM/FM radios with the addition of a second demodulator. Commonly, the tuner of a radio is used for the first demodulation phase. This base-band output of the radio""s tuner produces the main program audio with the sub-carrier superimposed. This signal is then fed into an additional frequency discriminator which finally xe2x80x98pulls outxe2x80x99 the SCA audio programming.
Commercial FM radio tuners are inadequate to perform frequency demodulation needed for the first phase of high quality SCA reception. This is for several reasons. First the filter circuitry is not matched for optimum reception from the antenna at its optimum length for the exact frequency of reception needed. This is impractical for commercial radio tuners to do because they need to cover the whole FM band from 88 MHz to 108 MHz. They can only have a general or somewhat compromised match to the antenna. Even higher cost radio tuners are not designed optimally for a given station with a varying length antenna. Rather it is designed for an external antenna with fixed impedance of typically 50, 75, or 300 Ohms.
Second, the tuners have a much wider front-end filter bandwidth. This allows more signals to enter the first mixer, which introduces intermods and spurious signals. The intermods are normally smaller and not an issue for the main programming audio. But they can create havoc with the lower level sub-carrier. The injected intermods can commonly be equal to or greater than the sub-carrier signal, degrading its quality.
Third, commercial tuners do not have high image rejection because they usually don""t need it for main carrier FM reception. Once an image frequency has xe2x80x98foldedxe2x80x99 over to into the IF portion of the first mixer, filtering can not remove it, because it would remove the desired portion at the same frequency. The sub-carrier is inherently more susceptible to this type of interference because of its lower level.
This patent submission provides an SCA tuner with optimized matching to the antenna at a pre-determined station frequency for the optimum antenna length. This provides for optimum signal transfer into the first mixer with low noise figure. If the telescopic antenna is collapsed down for strong signal areas, the circuitry has deliberate filter impedance mismatching; which reduces the signal gain and provides needed lower inter-modulation products for higher performance. This tuner design also has an exceptionally narrow filtering band-width into the first mixer, also minimizing any spurious or intermod interference. Furthermore the SCA tuner has extremely high image rejection to prevent interference of that nature into the first mixer. The second demodulation phase of the tuner has specific SCA filtering with a very high Q and with low group delay. These circuits work together to minimize xe2x80x98leakagexe2x80x99 of the main signal programming into the subsidiary signal. This is a common problem other sca designs, missing the features here listed. These features work together to create the highest quality output possible of the subsidiary signal.
An object of the present invention is to provide a SCA tuner circuitry for more optimal matching of the radio antenna when extended or collapsed, and to handle both strong or weak signal conditions.
Another object of the invention is to provide for weak signal inputs, a more matched filtering for minimal noise figure and maximal gain, while for strong signal inputs minimum gain and 3rd order intermodulation distortion products. A related object of the present invention is to engage unique filtering for RF image rejection having minimal impact to the required pass-band. The secondary demodulation phase of the tuner also rejects near-by interfering signals.
Another object of the present invention is to reduce main programming xe2x80x98leakagexe2x80x99 into the sub-carrier, yet minimize any xe2x80x98imagexe2x80x99 signal fold-over interference, while filtering out undesired out-of-band RF signal inputs.
Yet another object of the present invention is to have sharp RF front-end filtering that can be tuned to any selected frequency of the FM radio band and have minimal group delay distortion in the filtering components, while providing unconditionally stable front-end amplification, for a variety of source antenna impedances.
Still yet another object of the present invention is to provide narrow-band filters in a versatile way to selectively notch out undesired frequencies on the low side, while minimally affecting the pass-band.
This invention comprises unique circuitry for demodulating a sub-carrier radio signal. It optimally matches a signal coming from a commercial FM telescopic antenna, adjusting for the antenna being extended or collapsed. If the antenna is extended (as needed in a low level signal area) the signal is introduced to the first mixer with a low noise figure and maximum gain. If the antenna is collapsed (as indicated for a strong signal area) the signal is amplified with less gain and the intermodulation distortion generated is significantly reduced. In both cases the signal is selectively filtered in a narrow band-width which introduces only a minimum amount of group delay distortion. Also this circuitry can be adjusted or tuned across the complete FM commercial band.
This invention also comprises unique filtering circuitry, which effectively eliminates image signals into the front end of the tuner. This is done through a conversion process of the band-pass filter, which leaves the pass-band essentially unaffected. Also the tuner has a second demodulation phase requiring filtering, which has been similarly converted to notch out undesired lower frequency elements. It also leaves the pass-band in tact, and maintains very low group delay, essential for good sub-carrier detection.
This invention is intended to provide best quality demodulation of a subsidiary signal using a portable FM radio with a telescopic whip type antenna. To this end the following specific objectives are needed and realized in a unique producible fashion.
In accordance with a preferred embodiment of the present invention, in a radio receiver having an telescoping antenna and audio out line, an SCA radio turner comprises a matching filter between the antenna and a first FM discriminator to match the level of extension of the antenna; a phase-lock-loop circuit within a second FM discriminator following the first FM discriminator; and sub-carrier audio processing between the FM discriminator and the audio out line.